1. Field of the Invention
Generally, the present invention relates to a rubber composition containing EPDM rubber (ethylene-propylene-diene copolymer rubber) as its main component. More particularly, the present invention relates to a rubber composition which does not bloom vulcanizing accelerator in vulcanizing the vulcanizing accelerator-added EPDM rubber with sulfur so that paper-feeding rubber rollers, comprising the rubber composition, of laser beam printers, electrostatic copying machines, facsimiles and automatic deposit/payment machines (ATM) have a high friction coefficient and a high wear resistance.
2. Description of the Related Art
A rubber composition comprising the EPDM rubber (ethylene-propylene-diene copolymer rubber) contained therein as its main component has been hitherto used in large quantities for a paper-feeding roller of OA equipments such as laser beam printers, electrostatic copying machines, and facsimiles; and automatic deposit/payment machines (ATM). This is because paper-feeding rollers of the OA equipments and the automatic deposit/payment machines comprising the rubber composition are highly resistant to wear and are not deteriorated fast by ozone which is generated in the OA equipments when they are operated to form images. That is, the rubber composition is highly resistant to the ozone. In order to obtain a high coefficient of friction in the friction between the paper-feeding roller comprising the rubber composition and paper, softening agent such as process oil is added to the EPDM rubber to reduce the hardness of the paper-feeding roller. Further, in order to improve the wear resistance of the paper-feeding roller, oil-extended EPDM rubber is used to compose the rubber composition.
The EPDM rubber is used by vulcanizing it with vulcanizing agent such as sulfur or organic peroxide to increase its physical property (in particular, mechanical property). In a mixture material comprising the EPDM rubber, the softening agent such as the process oil, plasticizer, and the like, the reaction of the organic peroxide is inhibited by the softening agent. Thus, sulfur is practically used. The EPDM rubber does not have a double bond in its main chain. The double bond is present only in third components such as DCPD (clicyclopentadiene), 1,4-HD (1,4-hexadiene), and ENB (ethylidene norbornene) which are added to the EPDM rubber as a diene component. Further, the technique of polymerizing the third component with the polymer has a limitation. Thus, even though the EPDM rubber having the greatest iodine number (iodine number: 35) is vulcanized with sulfur, the vulcanizing speed thereof is lower than that of a diene polymer such as NR (natural rubber) or BR (butadiene rubber). In order to increase the vulcanizing speed of the EPDM rubber, vulcanizing accelerator has been hitherto added to the EPDM rubber in addition to the sulfur (vulcanizing agent) by selecting an appropriate kind of the vulcanizing accelerator and optimizing the addition amount thereof.
However, because the vulcanizing accelerator has a high polarity, it is difficult for it to be present in the EPDM rubber having a low degree of polarity. Thus, the vulcanizing accelerator is not dispersed uniformly in the EPDM rubber which has not been vulcanized. Consequently, in a product obtained after vulcanizing the EPDM rubber, the vulcanizing accelerator and reaction product thereof bloom. The blooming of the vulcanizing accelerator and reaction product thereof causes the reduction in the friction coefficient of the paper-feeding roller of the OA equipments and the automatic deposit/payment machine (ATM) formed by molding the product (EPDM rubber), into a roller shape, containing the vulcanizing accelerator and the reaction product thereof which bloom.
A method of suppressing the blooming by adding filler such as silica having a preferable adsorbing property to the EPDM rubber is known. The method is capable of suppressing the generation of the blooming of the vulcanizing accelerator and the reaction product thereof, but causes the hardness of the rubber composition to be increasingly high as the addition amount of the filler increases. Therefore, the paper-feeding roller comprising the EPDM rubber molded into a roller shape and containing much amount of the filler cannot be allowed to have a high friction coefficient for paper.
A method of suppressing the blooming of the vulcanizing accelerator by adding a small amount of polyalkylene glycol or polyoxyethylene alkylene ether to the EPDM rubber is known. But these methods are incapable of providing satisfactory blooming-suppressing effect and in addition, cause the polyalkylene glycol or the polyoxyethylene alkylene ether to be separated out from the vulcanized rubber composition.